The field of the invention is patient monitoring systems. More particularly, the invention relates to an improved patient monitoring method and system using pattern recognition techniques to eliminate noise points in an oscillometric envelope.
The heart muscles of humans periodically contract to force blood through the arteries. As a result of this pumping action, pressure pulses or oscillations exist in these arteries and cause them to cyclically change volume. The baseline pressure for these pulses is known as the diastolic pressure and the peak pressure for these pulses is known as the systolic pressure. A further pressure value, known as the “mean arterial pressure” (MAP), represents a time-weighted average of the blood pressure.
There are different techniques and devices for measuring one or more of these blood pressure values. One method in particular involves applying a pressure cuff about the upper arm of a human and inflating it above systolic pressure so as to restrict the flow of blood in the brachial artery. The pressure is then slowly relieved while a stethoscope is used on the distal portion of the artery to listen for pulsating sounds, known as Korotkoff sounds, that accompany the reestablishment of blood flow in the artery. As the pressure in the cuff is reduced further, the Korotkoff sounds eventually disappear. The cuff pressure at which the Korotkoff sounds first appear during deflation of the cuff is an indirect measure of the systolic pressure and the pressure at which these sounds disappear is an indirect measure of the diastolic pressure. This method of blood pressure detection is generally known as the auscultatory method.
Another method of measuring blood pressure is referred to as the oscillometric technique. This method of measuring blood pressure involves applying an inflatable cuff around an extremity of a patient's body, such as the patient's upper arm. The cuff is then inflated to a pressure above the patient's systolic pressure and then incrementally reduced in a series of small steps (or continuously at a substantially constant rate). A pressure sensor measures the cuff pressure throughout the blood pressure determination. The sensitivity of the sensor is such that pressure, fluctuations within the cuff resulting from the beats of the patient's heart may be detected. With each beat there is a resulting small change in the artery volume which is transferred to the inflated cuff causing slight pressure variations within the cuff which are detected by the pressure sensor. The pressure sensor produces an electrical signal showing the incremental cuff pressure and a series of small periodic variations associated with the beats of a patient's heart. It has been found that these variations, called “complexes” or “oscillations,” have a peak-to-peak amplitude which is minimal for applied cuff pressures above the systolic pressure. As the cuff pressure is decreased, the oscillation size begins to monotonically grow and eventually reaches a maximum amplitude. After the oscillation size reaches the maximum amplitude, the oscillation size decreases monotonically as the cuff pressure continues to decrease. Oscillometric envelope data such as this is often described to as having a “bell curve” appearance. Physiologically, the cuff pressure at the maximum value approximates the MAP. In addition, the complex amplitudes of cuff pressures equivalent to the systolic and diastolic pressures have a fixed relationship to this maximum value. Thus, the oscillometric method is based on measurements of detected complex amplitudes at various cuff pressures.
Blood pressure measuring devices operating according to the oscillometric method detect the peak-to-peak amplitude of the pressure complexes at the various applied cuff pressure levels. The amplitudes of these complexes, as well as the applied cuff pressure, are stored together as the device automatically changes the cuff pressures over a range of interest. These peak-to-peak complex amplitudes define an oscillometric “envelope” and are, evaluated to find the maximum value and its related cuff pressure, which is approximately equal to MAP. The cuff pressure below the MAP value which produces a peak-to-peak complex amplitude having a certain fixed relationship to the maximum value is designated as the diastolic pressure. Likewise, the cuff pressure above the MAP value which results in complexes having an amplitude with a certain fixed relationship to that maximum value is designated as the systolic pressure. The relationships of complex amplitude at systolic and diastolic pressures, respectively, to the maximum value, are empirically derived ratios which assume varying levels depending on the preferences of those of ordinary skill in the art. Generally, these ratios are designated in the range of 40% to 80%.
One way to determine oscillation magnitudes is to computationally fit a curve to the oscillometric envelope defined by complex amplitude data points which are measured by a blood pressure monitor at varying cuff pressures. The fitted curve may then be used to compute an approximation of the MAP data point, which is approximately at the maximum value of the fitted curve and is therefore easily determined by computing the point on the fitted curve in which the first derivative equals zero. From this maximum value data point, the systolic and diastolic pressures may be computed as fixed percentages of the maximum value. In this manner, the systolic data point and the diastolic data point along the fitted curve may each be computed and therefore their respective pressures may also be determined. This curve; fitting technique has the advantage of filtering or smoothing the oscillometric envelope. However, in some circumstances it has been found that additional filtering techniques used on the oscillometric envelope can improve the accuracy of the resulting blood pressure values.
The reliability and repeatability of blood pressure computations hinges on the ability to accurately determine the oscillation magnitudes of the complexes. There are several barriers to accurate and reliable oscillation magnitude determination. First, artifacts caused by patient motion and other effects are often present. These artifacts are superimposed upon the desired oscillation signal, causing it to be distorted. Second, many of the properties of the desired oscillation signal are not consistent from patient to patient, or even from oscillation to oscillation for a given patient. Because of these types of potentially adverse effects on the oscillometric signals, most automatic blood pressure instruments look for a consistency in pulses at a particular pressure level. For example, before a complex is accepted as adequate for use in the determination of blood pressure, there may be a requirement for consistency in pulse size compared to other pulses at the same step, adjacent steps, or previous determinations at the same pressure level. Further, there may be requirements on the consistency of pulse periods before a pulse is accepted as adequate for use in a blood pressure determination.
Despite significant signal processing efforts, artifact corrupted complexes are sometimes used in blood pressure calculations. Oftentimes, these complexes are of such an energy level that, they unduly influence the curve fit smoothing technique. Thus, there exists a need for a method and system for effectively using pattern recognition techniques to eliminate noise or physiologically unimportant points in oscillometric envelope data before it is used in calculating blood pressure.